1. Field of the Invention
The present application relates generally to magnetic resonance imaging (MRI) and nuclear magnetic resonance (NMR) spectroscopy, and more particularly to using an off-resonance preparatory pulse to enhance MRI sensitivity to O17-containing compounds in biological matters.
2. Description of Related Art
Magnetic resonance (MR) results in the absorption and/or emission of electromagnetic energy by nuclei or electrons, and provides the basis of some powerful imaging techniques in medicine. Magnetic resonance imaging (MRI) makes use of resonance properties in some of the nucleus, such as hydrogen nucleus (protons), in biological matters. Other nucleus that can be used for MRI include that of O17, which is a naturally occurring, stable isotope of O16 (spin 5/2) with a natural abundance (N.A.) of 0.038 atom %. O17 is detectable by MRI and is useful for assessing cerebral perfusion in animal studies using direct and indirect MR detection techniques. In the past two decades, several methods based on the use of O17 have been proposed for assessing tissue perfusion and functional metabolic imaging of regions with active oxidative metabolism. In nature, O17 is found to exist in two states, O17 gas and H2O17. Unlike H2O17, O17 gas cannot be detected directly using MRI. Thus, extensive research have been conducted to investigate the use of H2O17 as a potential in vivo contrast agent for perfusion studies.
H2O17 is a freely diffusible tracer and safe for human use. However, problems arise in the use of H2O17 for perfusion studies because of its low MR sensitivity. H2O17 also has a low natural abundance (N.A. is 0.038 atom %) and is thus costly to obtain. So, it is not cost effective for patient use if very large doses of H2O17 have to be injected in order to improve the MR sensitivity. Therefore, to detect small concentrations of H2O17 in tissue or biological matter is a prerequisite of an MR technique using H2O17. Conventional MR techniques, however, are either not sensitive to changes in small concentrations of tissue H2O17 or are not practical for use on clinical scanners.
Two types of MR techniques have been proposed to detect O17 in biological matters, one is based on direct detection of H2O17 in tissue using NMR spectroscopy, and the other based on signal changes due to effect of O17 on the proton signal (indirect detection). Due to low gyromagnetic ratio and short T2 relaxation times of O17, direct O17 NMR spectroscopy suffers from poor sensitivity at low H2O17 concentrations. See Pekar, J., et al., “In Vivo Measurement of Cerebral Oxygen Consumption and Blood Flow Using 17O Magnetic Resonance Imaging,” Magn. Res. Med.: 1991. 21(2): p. 313–9; and Fiat, D., et al., “In Vivo 17O NMR Study of Rat Brain During 17O2 Inhalation,” Magn. Res. Med.: 1992. 24(2): p. 370–4.
Some limitations of the direct techniques can be overcome by indirect methods, which are now more popular for MR imaging. See Hopkins, A. L., et al., “Oxygen-17 Contrast agents,” Fast imaging techniques, Investigative Radiology, 1988. 23 Suppl 1: p. S240–2. Also, some authors have proposed O17 decoupled 1H detection techniques using double tuned coil. See Reddy, R., et al., “17O-Decoupled 1H Detection Using a Double-Tuned Coil,” Magn. Res. Imag.: 1996. 14(9): p. 1073–8. A technique based on the O17 decoupled 1H detection techniques was successfully used to assess cerebral perfusion in animal cerebral ischemic model. See de_Crespigny, A. J., et al., “MRI of Focal Cerebral Ischemia Using (17)O-Labeled Water,” Magn. Res. Med.:2000. 43(6): p. 876–83. The above mentioned techniques, however, suffer from the disadvantage that some amount of hardware modifications to clinical scanners are required to successfully implement them. Thus, these techniques are not suitable for clinical use.
Recently Arai et al. proposed a T2w (T2 weighted) fast imaging technique, using multi shot Rapid Acquisition with Relaxation Enhancement (RARE) sequences, for assessing cerebral perfusion with bolus injection of H2O17 in animal model on a commercial clinical MR scanner. See Arai, T., et al., “Measurement of Local Cerebral Blood Flow by Magnetic Resonance Imaging: in vivo Auto Radiographic Strategy Using 17O-Labeled Water,” Brain Research Bulletin, 1998. 45(5): p. 451–6. In these article, the authors present that RARE based technique allows rapid detection of changes in brain tissue H2O17 concentration. MR techniques based on T1ρ (T1 rho) dispersion imaging have also been proposed for mapping H2O17 in rodent brain model. See Rizi, R. R., et al., “Proton T1rho-Dispersion Imaging of Rodent Brain at 1.9 T,” Journal of Magn. Res. Imag.: 8(5): p. 1090–6. One of the main advantages of these technique is that they can be easily implemented on commercial clinical MR scanners. Unfortunately, these techniques require high power RF preparatory pulses to obtain desired T1ρ contrast, which may result in increased power deposition in organic tissues and hence increased tissue heating, especially when multi-slice capability is engaged. To overcome the problem, another technique using low power off-resonance RF pulses was proposed by the same authors. See Charagundla, S. R., et al., “Off-Resonance Proton T1rho Dispersion Imaging of 17O-Enriched Tissue Phantoms,” Magn. Res. Med.:1998. 39(4): p. 588–95. In this proposal, experiments were performed to characterize T1ρoff off following off-resonant spin locking in H2O17 enriched phantoms. This off resonance technique, however, was shown to be no better than the on resonance spin locking technique in demonstrating contrast between H2O17 enriched phantoms.